\(\omega _{\vec{k}}^{2}(\eta )=\vec{k}^{2}+m^{2}a^{2}(\eta )\) , and `dot' represents derivative w.r.t the conformal time \(\eta \) . In the leading order adiabatic approximation [1]}, [2]}, [3]}, we can write \(W_{\vec{k}}(\eta )=\omega _{\vec{k}}(\eta )\) .
